As a gas exhaust pump capable of high-speed and longtime continuous operation, there is conventionally known, for example, a positive displacement screw pump having a pair of screw rotors in a stator (see NPL 1). Recently, there is a need for establishment of techniques for manufacturing gas exhaust pumps with variable-lead/variable-inclination-angle screws at a large scale and for commercialization by use of the techniques for the lower cost of the gas exhaust pumps with variable-lead/variable-inclination-angle screws since they have a pumping capability of a wide range from a molecular flow region to a viscous flow region, a constant pumping speed irrespective of the type of gas, and a high ultimate pressure.
Meanwhile, various types of pumps are used in an apparatus for manufacturing display devices which use semiconductor devices, liquid crystal, organic EL, and the like, and functional devices such as solar cell devices, due to limitations of application ranges depending on the pumping performance. Since the above-mentioned pump has a wide range of application of decompression and the pumping performance does not depend on the type of exhaust gas, there is no need to perform complicated works, such as replacement of a pump depending on the type of gas, placement of a pump in accordance with a change in pressure conditions, or preparation of a pump suitable for each pumping position in a production system having a plurality of pumping positions. If the use of a pump does not depend on a pumping speed, the same type of pump can be used, thereby eliminating troublesome selection of a pump for each pumping position. If the above type of pump becomes commercially available at low costs, it can be easily expected that such a type of pump will become widely popular and greatly contribute to the development of the industry.
FIG. 1 is a schematic view of an exemplary pump of the above-mentioned type. FIG. 2 is an enlarged schematic view of a portion shown by II in FIG. 1. A gas exhaust pump 100 with variable-lead/variable-inclination-angle screws includes an variable-lead/variable-inclination-angle female screw rotor 101 and an variable-lead/variable-inclination-angle male screw rotor 102. A screw engaging portion 104 is formed between the screw rotors 101 and 102, in which teeth and grooves are engaged with each other with a predetermined clearance to obtain a safe and smooth rotary motion. When the female and male screw rotors 101 and 102 are fixed to their rotating shaft (a rotating shaft of the female screw rotor 101 is not shown; a rotating shaft of the male screw rotor 102 is a rotating shaft 105), their engagement conditions are maintained. The screw rotors 101 and 102 are installed in a stator 106 with a predetermined gap provided between tooth top ends of the screw rotors 101 and 102 and an inner wall of the stator 106.
The rotating shaft 105 is rotatably mounted to a bearing body 116 via a holding means such as an angular bearing 107 (FIG. 1 shows four angular bearings 107a, 107b, 107c, and 107d for convenience). The male screw rotor 102 is fixed to the rotating shaft 105 and is rotated by the rotation of the rotating shaft 105. A lubricating oil supply path 109 is provided in the rotating shaft 105. A lubricating oil 111 is stored in a lubricating oil reservoir 112 provided at a predetermined position under a base plate 110. When the rotating shaft 105 receives a rotational force of a motor (not shown) via a rotary gear (not shown) and rotates, the rotation generates a centrifugal force so that the lubricating oil 111 rises by suction through the lubricating oil supply path 109 to be supplied to the angular bearing 107.
An oil seal member 113 for preventing the lubricating oil from diffusing is provided all around the rotating shaft 105 so as to seal a gap between the rotating shaft 105 and a seal housing 108 (they form an axis seal mechanism) as shown in FIG. 1 so that the lubricating oil 111 does not diffuse into a portion other than the angular bearing 107 through the gap between the rotating shaft 105 and the seal housing 108. However, providing only the oil seal member 113 may not be sufficient. Accordingly, a seal gas such as N2 is supplied to the gap between the rotating shaft 105 and the seal housing 108 through a seal gas supply path 114 as shown by arrows in FIG. 1 to prevent the lubricating oil itself or its vapors from diffusing upstream of a vacuum system. The seal gas is supplied from the seal gas supply path 114, flows through a predetermined passage, and is discharged outside from a discharge path (not shown) with other gases used in semiconductor processes such as film deposition and etching.
As shown by the screw engaging portion 104, the female and male screw rotors 101 and 102 are engaged with each other. More specifically, a top end surface of a tooth of one screw rotor (a top end surface 201 of a tooth of the screw rotor 102) is engaged with a bottom end surface of the other screw rotor (a bottom end surface 202 of a groove of the screw rotor 101, which corresponds to a bottom end surface 202 of a groove of the screw rotor 102) with a small gap therebetween so that the screw rotors can smoothly rotate.
In the case of a pump including female and male screw rotors having a structure in which rotation is transmitted from one screw rotor to the other screw rotor via a gear or the like, in an engaging portion between the teeth and grooves of the female and male screw rotors, an inner surface of the groove of one screw rotor is generally configured to face an outer surface of the tooth of the other screw rotor with a small gap therebetween so as to maintain smooth rotation of the screw rotors.
In the case of a pump configured to transmit a rotation driving force of a rotation driving source, such as a motor, from the rotation driving source to a first screw rotor via a gear or the like and transmit the rotation driving force from the first screw rotor to a second screw rotor via an engaging portion, a side surface of a tooth and groove of the first screw rotor smoothly contacts a side surface of a tooth and groove of the second screw rotor so that the rotation driving force is smoothly and efficiently transmitted to the second screw rotor.
The screw pump of FIG. 1 has a pair of (twin) screw rotors. There is also a screw pump having a single screw rotor and configured to rotate the screw rotor for pumping in a state where a gap is provided between a top end surface of a tooth of the screw rotor and an inner wall surface of a stator (see PTL 1).